Sintering plants produce valuable feedstocks for blast furnaces from fluxes, ore fines and various ferrous waste materials such as blast furnace flue dust and oily mill scale. The gases emanating from the sintering plant windboxes contain entrained particulate matter, soluble solids, the acid gases SO.sub.2 and CO.sub.2 and condensable hydrocarbons. Sinter plant windbox emissions have proven difficult to clean economically, particularly in the face of the escalating cost of energy.
Mechanical gas cleaners were initially used for removing particulates from sinter windbox emissions. Subsequently, mechanical gas cleaners were generally replaced with dry electrostatic precipitators. Because of the failure to remove condensable matter which escaped as fume from dry electrostatic precipitators, wet collection devices were then developed. Wet venturi scrubbers are efficient gas cleaners, but they operate with a high consumption of energy. Accordingly, attention has focused on wet electrostatic precipitators (WESP) for they can adequately clean the sinter windbox gas streams at a low level of energy consumption. Several commercially available WESP systems, as an added feature, incorporate a pre-scrubber spraying section ahead of the high voltage electrostatic precipitator section.
However, several problems have prevented the successful utilization of WESP systems for sinter plant emissions control. Firstly, when a WESP system is operated with once-through wash water, the enormous quantity of contaminated water for disposal (blowdown) has a total dissolved solids content that may be in excess of that permitted by wastewater regulations even though the concentration of dissolved solids in the water is low. Also, this huge volume of spent wash water would either overtax the capacity of present water treatment plants or require construction of very large treatment plants. Secondly, while recycling of the WESP system water will allow a minimum discharge and water treatment, recirculation creates an acidic, corrosive environment (for example, pH3; 500 ppm chloride ion concentration) within the apparatus that is unacceptable to continuous trouble-free operation.
In order to minimize corrosion of wet gas cleaning apparatus due to the acid nature of the recycled system water it has been customary to neutralize the recirculating water with caustic soda or the like. Since calcium and magnesium, which are present in the sinter strand mix, are being removed from the windbox gas stream by the WESP system and are accumulating in the recirculating water, neutralization of the recirculating water with caustic to avoid an acidic liquor results in the deposition of calcium and magnesium carbonates within the apparatus. To maintain the discharge water at pH 7, the water entering the WESP must be at least pH 10. The combination of the accumulating calcium concentration and this neutral to slightly basic environment dictates that the solubility of calcium carbonate is eventually surpassed and undesirable scale deposits will form.
Additionally, recirculation of the water means recirculating the particulate matter extracted from the gas stream with the inevitable clogging and plugging of the water sprays or water distributors that provide the collecting electrodes of the WESP with a cascading flow of washing water. This problem is particularly pronounced in WESP systems having low pressure water distributors.
Accordingly, there is a need for the capability to treat sinter plant windbox gases so as to meet stringent air and water pollution codes while minimizing the expenditure of energy.
In addition, there is a need for a WESP system for sinter plant emissions that can minimize the discharge of wastewaters from the WESP by recirculating acidic washing water.
There is a further need for a WESP system that recirculates the water without obstructing or stopping the water sprays or distributors with particulate matter extracted from the treated gases.